In principle there are two main types of drive trains used in wind turbines. The first type, as shown in FIG. 1, is the more traditional type of turbine comprising a gearbox attached between a main rotor shaft and a generator of the wind turbine. The second type is a gearless type, where the gearbox and the conventional generator are substituted by a multi-polar generator, or so-called direct drive generator.
The size and design of a direct drive generator is influenced by its rotational speed and by the desired power output. Assuming constant rotational speed, the two primary factors in power output are the amount of active material and the radial distance of the active material. Accordingly, for a desired or optimal power output it is possible to reduce the necessary amount of active material by increasing the diameter of the direct drive generator. Thus, it is advantageous for direct drive generators with low rotational speeds, such as direct drive generators used in hydro and wind turbines, to have a large outer diameter.
While the large outer diameter has advantages, it also causes numerous issues for direct drive generators. Among other problems, the large outer diameter causes problems with transporting, lifting, assembling, repairing, and replacing the direct drive generator. These problems are due not only to the size of the device (which significantly impacts transport) but also the weight of the direct drive generator. For example, for typical rotating electric machines, the air-gap (the distance between rotor and stator) is kept no larger than a few millimeters in order to avoid excessive magnetization requirements. In order to closely maintain this air-gap, in conventional rotating electric machines the rotating and stationary parts of the machine are relatively stiff, structurally, on an individual basis. In addition, the support structure, commonly a bearing system is also very stiff. An additional constraint is that the components that affect the shape and dimensions of the parts facing the air gap between the rotating and stationary parts must be produced and assembled with a high degree of precision as the nominal air gap of such machines is typically less than 0.375″ on a diameter that can approach 144″. Additionally, the heavy weight of a multi-megawatt generator of conventional design necessitates additional infrastructure support in the turbine tower and related components, which adds cost and complexity.
As partial solutions to these problems, there have been suggestions on dividing the parts of the generator:
In WO 98/20595 A1 a stator for a rotating electric machine is disclosed comprising a stator core and a winding. The stator core is provided with stator teeth extending radially inwards towards a rotor. Each stator tooth is configured as a number of tooth sections joined axially into a stator tooth plank. That stator tooth planks are fitted together side-by-side thus forming a section of the stator core. This construction makes the transport of parts of the rotating electric machine to the site of erection partially easier because the stator can be assembled on site. However, this construction requires a stator housing having relatively large outer dimensions.
From U.S. Pat. No. 4,594,552, an armature winding of a split stator is known. The split stator has a slotted core divided by at least two circumferentially-spaced split lines to facilitate the assembly and the disassembly of the split stator. The armature winding comprises armature coils in the slots of the stator core connected to provide poles and arranged to provide a plurality of armature coils divided at the split lines. Connecting and disconnecting means are provided to connect and disconnect the armature coils when the split stator is assembled and disassembled, respectively. This construction, however, also requires a stator housing having relatively large outer dimensions.
U.S. Pat. No. 5,844,341 describes an electric generator to be driven by a low speed device such as wind turbine. The generator consists of one or more rotor rings of many permanent magnets of alternating polarity and coaxial stator rings of many laminated yokes, each yoke defining slots to locate coils. The yokes and coils form modules which are supported by beams relative to the rotor rings. The drawbacks of this configuration are that the electromechanical properties in this form of modular construction, with single polar pairs separated by air gaps, may be disadvantageous; that a possible dismantling of a single stator module can require that the whole generator has to be opened in situ thus implying risk of humidity, dirt, etc.; and that it may be cumbersome if the stator module has to be taken out in a disadvantageous direction.
U.S. Pat. No. 6,781,276 B1 describes a generator for a wind turbine comprising a stator and a rotor. The stator has a number of stator modules that are individual and which may be installed, repaired and dismantled individually and independently of each other. This generator has no part larger than the air gap diameter. But even if no part is larger than the air gap diameter, the largest element to be transported still has a substantial size, given that the rotor is a single piece. In its completed form, this rotor is fitted with strong permanent magnets and needs to be covered by a nonmagnetic layer, e.g., wood or polystyrene of a certain thickness, during transportation. And while the dimensions of the rotor are smaller than the dimensions of the finished generator, it is still, at maybe 5 m diameter and 1.5 m length, a very substantial piece of equipment to transport.
U.S. Patent Publication No. 2009/0134629 describes a generator for a wind turbine comprising ring-segmented-shaped stator segments and ring-segment-shaped rotor segments. The stator and rotor are separated by a radial air gap wherein each ring-segmented-shaped stator segment is located radially outward from the ring-segment-shaped rotor segments. By using a ring shaped stator and ring shaped rotor, the invention requires axial width in order to provide the amount of active material desired at a certain diameter. Some further reduction in weight is described by reducing the supporting structure that limits the axial movement of the rotor and stator.
Although present devices are functional, they are not sufficiently efficient, effective or otherwise satisfactory. Accordingly, a system and method are needed to address the shortfalls of present technology and to provide other new and innovative features.